There has been a device control system, in which an operation terminal is connected to a network and multicasts or collectively broadcasts a control command to a plurality of control devices on the network, thereby controlling the plurality of control devices. The control command is a broadcast packet for indicating control contents to the plurality of control devices. Then, each control device responds by transmitting a state change notification, including a control state data, to the operation terminal, when a control state is changed by executing the received control command.
However, in the collectively broadcast, when the plurality of control devices respond by transmitting state change notifications at the same time, the packets of the state change notifications may collide with each other, in the case where the communication transport in the network is performed at low speed. Therefore, there is a possibility that the packet loss is generated and the operation terminal can not receive the state change notifications.
And so, a device control system as disclosed in Japanese Patent Application Publication No. 3-250934 has been proposed. First, when an operation terminal collectively broadcasts a control command to a plurality of control devices on a network, each control device executes the received control command. In this case, even if the control state is changed by the execution, each control device does not transmit the state change notification to the operation terminal. The operation terminal performs the unicast transmission of a state notification request command sequentially with respect to each of the plurality of control devices individually, after the broadcast of the control command. Each control device transmits the state change notification to the operation terminal, when receiving the state notification request command. That is, after the broadcast of the control command to the plurality of control devices, the operation terminal performs polling to each control device, thereby receiving the state change notification from each control device.
In the device control system as disclosed in the above document, the operation terminal has need to perform the unicast transmission of a state notification request command sequentially with respect to each of the control devices individually, after the broadcast of the control command. Therefore, the communication traffic on the network is increased.
Further, other operation terminals, except for the operation terminal that performs polling after the broadcast of the control command, can not receive the state change notification from each control device. Therefore, these operation terminals can not get the change information of the control state.